1. Field of the Invention
The present invention relates to self-adhesive silicone compositions which can be crosslinked to form elastomers and also their use. The invention includes silicone elastomers and material composites produced from the self-adhesive addition-crosslinking silicone compositions of the invention.
2. Description of the Related Art
It is known that platinum-crosslinking silicone rubber formulations which are brought into contact with customary substrate materials such as glasses, metals or organic plastics and subsequently vulcanized have only a low, if any, adhesive strength, so that the silicone elastomer obtained can generally be delaminated again from the substrate material concerned by application of relatively low tensile forces.
Numerous technologies for producing a strong and lasting bond between a silicone elastomer and a substrate material are known from the literature.
In principle, it is possible to modify the chemical and physical nature of the substrate material in order to improve the adhesive strength between the silicone elastomer and the substrate material. An illustrative method is pretreatment of the surface of the substrate materials by flaming, corona treatment, plasma treatment or Pyrosil® treatment. In such pretreatment steps, the surface or layer close to the surface of the substrate material is physically or chemically activated, i.e. functional groups which make formation of a bond possible and in this way contribute to achievement of a lastingly stable hard-soft material composite of silicone elastomer and substrate material are created.
Another way of producing lastingly strong material composites is application of primers to the substrate material. Such primers comprise adhesion-promoting additives and also solvents which have to be removed again after application of the primer to the substrate material.
A further possible way of producing lastingly strong composites is the provision of suitable functional groups in the volume or on the surface of the substrate material, which groups contribute to greater adhesion on vulcanization of the addition-crosslinking silicone compositions. On this subject, reference may be made, for example, to EP-A 601 882 and EP-A 143 994.
A critical disadvantage of the technologies described is that at least one additional process step for pretreatment or production of the substrate materials is necessary, and this is again undesirable in principle because of the comparatively low productivity and the higher process costs associated therewith.
For this reason, numerous approaches by means of which an improvement in the adhesion on various substrates can be brought about by use of one or more specific bonding additives and/or particular crosslinkers in the case of addition-crosslinking silicone compositions, have been described in the past. These additives which are mixed into the uncrosslinked compositions typically bring about an increase in the adhesion to a substrate material during vulcanization by action of heat or after vulcanization, possibly only after storage. Examples of these may be found, by way of illustration, in WO-A 09/037,156 and the further references cited therein.
However, the use of these specific bonding additives or specific crosslinkers is disadvantageous for a number of reasons.
The organofunctional alkoxysilanes/alkoxysiloxanes frequently used for increasing adhesion eliminate alcohol during vulcanization. Preference is typically given, for reactivity reasons, to methoxysilane derivatives which liberate toxic methanol. Along with the liberation of volatile dissociation products (elimination of alcohol), a not inconsiderable shrinkage of the silicone elastomer, which is generally undesirable, is also observed. The alcohol eliminated can also work against good adhesion since the alcohol accumulates at the surface of the silicone and therefore also at the interface to the substrate, as a result of which contact between silicone and substrate surface is reduced. In addition, in the case of functional alkoxysilanes, there is a risk of “efflorescence” and “sweating”.
Furthermore, it is known that the addition of particular bonding additives can have an adverse effect on the rheology of silicone compositions. Typical effects are thickening through to setting or complete stiffening of the unvulcanized formulations, which can significantly restrict their fitness for use, in particular for coating, casting or encapsulation processes.
The bonding additives used are frequently only effective under the action of heat, so that a particular minimum vulcanization temperature has to be employed. However, this narrows the possible fields of use of the silicone compositions concerned, in particular when industrial adhesive bonding or lamination processes aim at processing at room temperature.
Owing to their chemical constitution, the additives or crosslinkers used are slightly incompatible with the remaining constituents of the silicone composition and thus accumulate at the phase interface silicone/substrate during vulcanization. However, this thoroughly desirable, intrinsic micro-phase separation is associated with the silicone compositions in question not being clear and transparent but milky to opaque. Highly transparent, self-adhesive silicone elastomer compositions cannot be obtained in this way.
Some of the additives indicated can, owing to their relatively high reactivity, bring about accelerated SiH degradation, significantly reduce the crosslinking rate or even lead to (irreversible) inhibiting effects. In addition, in many cases the storage-stable production of some very specific bonding agents or bonding agent polymers is very complicated and therefore costly, which ultimately adversely affects the economics.